The present invention is related to capacitors and more particularly to a structure and method of providing a capacitor having a low resistance electrode including a thin layer of silicon.
The capacitance of a capacitor is determined not only by the materials that make up the capacitor, but also primarily by the thickness of the capacitor dielectric and the dimensions of the capacitor plates according to the equation for an ideal capacitor below:C=kA/d  (1)where C is the capacitance, k is the dielectric constant of the capacitor dielectric, A is the area in which the capacitor plates overlie each other, separated by the capacitor dielectric, and d is the thickness of the capacitor dielectric.
It is generally desirable to fabricate a capacitor having a very thin capacitor dielectric in order to maximize the capacitance, since in the equation (1) above, the capacitance varies inversely with the thickness of the capacitor dielectric.
The capacitor's electrical properties also depend to some extent upon the operating environment, that is, the manner in which the capacitor is used. The ranges of frequency, voltage and temperature in which the capacitor is operated all play a role in the way that the capacitor behaves in a circuit.
In order to obtain a capacitor having properties desired for a particular application, fabrication within tolerances for manufacturing, performance and reliability is critical. This is particularly true with respect to capacitors that are utilized in high frequency circuits such as radio frequency (RF) circuits.
Other factors that affect the performance of a capacitor include equivalent series resistance of the capacitor plates and the breakdown strength of the capacitor dielectric. All capacitors exhibit some resistance between their terminals, known as equivalent series resistance (ESR). ESR increases with the switching frequency of the voltage applied to the capacitor. As an example, a capacitor can be connected in a circuit in series with a load resistance to provide a high-pass filtering function. In such case, high equivalent series resistance (ESR) is undesirable, since it causes the signal voltage to be resistively divided relative to the load resistance. In such manner, high ESR can diminish the degree to which signals within the upper frequency passband are distinguished from lower frequency signals below the passband.
The dielectric breakdown strength is a primary measure of both the expected lifetime and reliability of a capacitor, as well as its ability to tolerate voltage spikes and electrostatic discharges which can cause sudden failure of the capacitor.
High capacitance per unit area and high dielectric breakdown strength are in apparent conflict. To increase capacitance per unit area a thinner capacitor dielectric is required, but a thinner capacitor dielectric generally results in lower dielectric breakdown strength.
Conventionally, dielectric films of capacitors are deposited by low temperature processes, i.e., processes conducted at temperatures below about 400 degrees C. For example, chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD) are frequently used to deposit a nitride dielectric film or an oxide dielectric film onto a metal capacitor plate at a temperature below 400 degrees C. Low temperature deposition processes such as CVD and PECVD can be used to deposit a dielectric film on a wide range of capacitor plate materials, including metals such as copper and aluminum. The use of copper and aluminum as capacitor plate materials is desirable because the metals have better conductivity than other metals.
However, the dielectric films that are deposited by low temperature processes tend to have defects, such as pinholes, and include impurities. These defects are brought about by the inclusion of gaseous species in the dielectric film during the deposition. Such defects lower the dielectric breakdown strength of the film, impacting reliability. To compensate for the lowering of the dielectric breakdown strength, the thickness of the capacitor dielectric can be increased. However, this is undesirable, because the capacitance then decreases across the thicker dielectric layer.
Dielectric films which are deposited by processes at high temperatures have properties which are superior to those deposited by low temperature processes. High temperature processes include processes performed at temperatures over about 400 degrees C., such as low-pressure chemical vapor deposition (LPCVD), which is typically performed at temperatures ranging between 700 and 900 degrees C. Such high temperature processes tend to produce more uniform dielectric films which are less prone to the inclusion of pinholes and impurities than low temperature processes, resulting in higher dielectric breakdown strength per unit of thickness than low temperature deposited CVD and PECVD films.
However, the dielectric films that can be deposited by high temperature processes limit the choice of materials for the underlying capacitor plate. Such capacitor plate materials must either not react at all with reagents present in the high temperature process or react only beneficially to them. Moreover, the capacitor plate materials must be able to withstand the high temperatures and subsequent patterning steps of the process. For example, a capacitor plate of tungsten does not serve as a suitable surface on which to deposit a dielectric layer of silicon dioxide or silicon nitride by LPCVD because oxygen ions present in the deposition chamber tend to oxidize the exposed tungsten, severely affecting its conductivity.
In addition, in order for the capacitor to perform well at relatively high frequencies including radio frequencies up to microwave frequencies, the choice of capacitor materials is further limited to the choice of those materials for the capacitor plates and the dielectric film that produce lower thermal losses and/or lower radiative losses than other materials. Thermal and radiative losses can be produced in a capacitor when the energy of AC cycling between voltages is converted to heat and radiation from the capacitor. The selection of low loss materials is desirable in order to maintain the equivalent series resistance of the capacitor within tolerable limits. Certain capacitor dielectrics materials, such as tantalum pentoxide and aluminum oxide, while being attractive because of their very high dielectric constants, nevertheless produce high thermal losses which are manifested as a high equivalent resistance of the capacitor. Other high dielectric constant materials such as ferroelectric dielectric materials, e.g. perovskite materials including but not limited to strontium titanate and barium strontium titanate, cannot be placed in direct contact with certain metals such as copper, aluminum and tungsten, because of harmful interaction that can occur to ultimately destroy the dielectric film.
A particular type of capacitor known as a “metal-oxide-semiconductor” (MOS) capacitor is formed as an element of an integrated circuit chip. The MOS capacitor has one electrode provided by a doped single-crystal semiconductor region of the chip, a capacitor dielectric in contact with the single-crystal semiconductor region and another electrode formed by depositing silicon or a compound of silicon (i.e., a silicide) over the thin dielectric. The thin dielectric typically is formed by thermal oxidation at the surface of the single-crystal semiconductor region.
MOS capacitors typically are utilized to provide large on-chip capacitance, e.g. for decoupling at desired locations within the chip. MOS capacitors are used for on-chip capacitors because they can be made in the chip forming process. However, MOS capacitors are generally undesirable for off-chip applications. In an MOS capacitor, the lower electrode typically is formed in an n+ doped well at the surface of a p-type single-crystal semiconductor substrate. A junction capacitance exists between the n+ well at the surface and lower p-type area of the semiconductor substrate. This junction capacitance, being in series with the capacitance of the MOS capacitor, effectively reduces the capacitance of the capacitor. In addition, processes for making MOS capacitors cannot be optimized for the properties of the capacitor. This is because processes for making MOS capacitors must be integrated into other processes, e.g. implants, gate oxidation and gate conductor fabrication, which are performed concurrently to fabricate transistors on the chip.
A capacitor structure and method of making the same are needed by which one or more of the following properties are obtained: high capacitance per unit area, high dielectric breakdown strength, reduced thermal losses and reduced equivalent resistance.